Downsides to salt pools

[chem geek]

Brad,

I can only refer you to this same EPA document that I referenced before where it says that the 316 stainless steel is more resistant to chlorine pitting than 304. However, it does show in at least some studies that 316 is superior to 316L, but I would find it surprising that salt levels above 500 ppm are simply corrosive to 316L. The chlorine appears to be far more corrosive to stainless steel than the salt level, though the salt level may accelerate such corrosion. So my take would be that using CYA would cut down the corrosion from chlorine to the point where the higher salt levels of an SWG pool wouldn't be a problem with 316 type steel.

However, this is all just speculation on my part based on the fact that we haven't had any reports of stainless steel corrosion in SWG pools that have been using CYA (and haven't had horribly high chlorine levels). We did have the one report of the indoor pool with 3-5 ppm FC, but that had no CYA.

So if your customer is up to it, they can bring up the CYA level in their pool to at least the 10-20 ppm range (if not 30 ppm) and then try the SWG (with the 3000 ppm salt). My prediction is that they won't see corrosion. If they had 304 stainless steel, then they *might* see corrosion eventually, but even then I'd bet it would take a few years.

Now having CYA in an indoor pool is going to present a different set of problems since indoor pools tend to get combined chlorines that do not break down as readily as outdoor pools (due to the lack of sunlight) so you might need to periodically use a non-chlorine shock (potassium monopersulfate) for the indoor pool. First try monitoring (or have the customer monitor) the combined chlorine level -- if it's not a problem, then you're done. If not, then use the non-chlorine shock.

By the way, you mentioned that the customer saw corrosion issues with the walls and floor of the pool. What does that mean? Isn't the pool plaster/gunite? Is he now talking about corrosion as in roughness of the plaster finish? Or is this pool made out of stainless steel?! Or do the walls have some stainless steel bars and by "the floor" he means metal floor drains? I'm confused.

That's my two cents (again, for what it's worth),
Richard

[Waterworks]

The pool is a one-piece shell made entirely of 316L stainless steel and was set in place by a crane; like a one-piece fiberglass pool, but made of steel. Don't ask how much it cost because I have no idea. I won't be able to convince him to try the SWG on that pool again because if I'm wrong it's an awfully expensive mistake. After the first round of corrosion the metalurgist applied some sort of clear corrosion proof sealant and somehow got rid of the original corrosion. I'm guessing that this was expensive as well.

Brad

[chem geek]

Brad,

Yikes! I had no idea such a thing as an all stainless steel pool existed. You are absolutely right that you don't want to take any chances with that kind of pool. Not using salt and adding CYA would be wise, just to play it absolutely safe. Since the customer is using Trichlor tabs (pucks), you should remind him that it is important for the feeder (if it's a floating feeder) not to park itself near the side of the pool with the circulation off. Otherwise, the acidity from the Trichlor will corrode the stainless steel in the vicinity. This is what happened to me in my own pool with stainless steel bars along the side of the pool -- the bars didn't corrode, but the mountings closest to the floating feeder did.

Richard

[Waterworks]

I don't think that Stainless Steel pools actually do 'exist' per se. As far as I know one of his companies builds SS tanks for agriculture or something and he had them build him this one as a one-of-a-kind thing.
He uses an off-line chlorinator, so the floating thing won't be a problem.

Brad

[chem geek]

Brad,

OK, so with an off-line chlorinator he won't have the problem of the floating dispenser, but given this guy's history of not monitoring his water chemistry (which may have gotten the chlorine to a high level) he could now be in the position of getting his pH very low in his pool if he uses Trichlor exclusively and doesn't check the pH once and a while and add pH Up (Sodium Carbonate or Washing Soda). If the pH gets too low, then he can end up corroding his stainless steel again. Given his history, it would be a good idea to keep the TA on the high side, at least 120 ppm, as long as he's using Trichlor. Also keep in mind that with the CYA from the Trichlor he will have to maintain higher Free Chlorine levels as the CYA level rises in order to have proper disinfection and to prevent algae. See Ben's Best Guess CYA chart as a rough guide for how much FC is needed at various levels of CYA.

By the way, if you can get contact info for the metallurgist, then you can give that to me (via E-mail or Private Message) and I'll contact him to see where he got his 500 ppm salt limit for this type of stainless steel.

Thanks,
Richard

[Poolsean]

There's a company in North Carolina, Bradford Industries, that makes these Stainless Steel pools and spas and cost into the Tens of thousands of dollars! Most of the ones they display and are on their brochures show most of the pool/spa being tiled. However they do have some very large custom built ALL Stainless Steel commercial projects.

Very unusual looking with all straight sides. We have one of these all stainless steel spas in Boca Raton Florida at what was formerly the Mission Bay Aquatic Complex. This is a non-salt chlorinated spa. They use sodium hypochlorite from a Stenner Chemical Feed Pump. Most of the corrosion appear at the welds.

Just the fact that this spa was already showing signs of corrosion was reason enough for me to not propose a salt system on it. I'm sure salt would have been blamed for any more damage.

[Waterworks]

This guy also has four pure copper spas from Diamond Spas at the same residence. AFAIK these spas are similar to Bradford spas' units. There is some tile and some copper exposed. He uses inline chlorinators for these units and has not complained at all about corrosion.

Brad

[chem geek]

That's interesting about the copper not showing signs of corrosion while the stainless steel did. I wonder if the chlorine levels are better maintained in his spa or if copper is more resistent to any potentially salt-amplified chlorine corrosion. Copper is more resistant than steel so perhaps even though chloride ion might interfere with any protective oxide layer that gets formed, that once removed the underlying steel in stainless steel corrodes faster than the underlying copper under any thin copper oxide protective layer (if any). This is all speculation on my part.

On a separate topic, I ran into this article about the damage to plaster from high levels of CYA. What is interesting is that the drop in CYA over time seems to indicate that even levels of CYA not much lower than 100 ppm can be detrimental to plaster. If you look at the line that starts with 200 ppm, it drops rather quickly down to around 130 ppm and then continues to drop more slowly to around 80-90 ppm. This may indicate that there is some plaster deterioration effect even at 80 ppm though it is clearly much slower. This would be another reason for SWG manufacturers to take a look at seeing if they can't figure out a way to operate more efficiently at lower CYA levels.

Richard